The present invention pertains to a process for measuring and checking concentration values of a plurality of gas components in a gas sample.
A gas-measuring device with a plurality of sensors, which are provided for the measurement of the gas components to be expected and whose measuring sensitivity is selected correspondingly, is used, in general, for the measurement in a gas sample. The gas sample consists of individual gas components, which are simultaneously detected by the sensors.
A frequent problem in gas measurement by means of the prior-art gas-measuring device is the insufficient selectivity. This means that the sensors have cross sensitivities for other gas components besides the desired sensitivity for the particular gas component to be measured. The occurrence of other gas components, with respect to which a sensor has cross sensitivity, leads to distorted measuring results, which cannot be compensated if they are not recognized as such. Additional distortions of the measuring results occur, e.g., due to variations in temperature, moisture effects, contamination of the gas-measuring device, aging-related sensitivity drift and zero drifts of the sensors.
One possibility of recognizing such disturbing effects on electrochemical gas sensors during the measurement of concentration values of gas components in a gas sample and of compensating them as a result is to use an additional measuring electrode. An electrochemical multiple-gas sensor, with which different gas components in one gas sample are detected simultaneously, is described in DE 196 22 931 A1. A measuring electrode for hydrogen sulfide, which consists of gold, iridium and carbon or graphite and is to detect a cross sensitivity of the other measuring electrodes to hydrogen sulfide, is additionally provided there.
The process according to the present invention represents a further improvement besides the use of an additional measuring electrode in an electrochemical gas sensor for recognizing disturbing effects. The process can also be applied to gas sensors that are based on different principles of measurement, e.g., infrared optical gas sensors, semiconductor gas sensors or catalytic heat tone gas sensors.
The object of the present invention is to provide a process with which the occurrence of disturbing effects due to a great variety of causes, e.g., temperature variations, moisture, contamination, as well as drift phenomena, can be recognized on the sensors.
A gas-measuring device with n sensors, where n is the number of sensors and m n equals at least 2, is used for the process for measuring and subsequently checking concentration values of gas components in a gas sample. Furthermore, the gas-measuring device comprises an evaluating unit, which receives and evaluates the values measured and passed on by the n sensors.
A plurality of different calibrating gases are introduced into the gas-measuring device with the n gas sensors for measuring the concentrations of m less than n possible gas components.
Gases that comprise the gas components that are later taken into consideration during the measurement in the gas sample are used as calibrating gases. Gases that comprise a single possible gas component are advantageously used as calibrating gases; this leads to m different possibilities, but it is also possible to use gases that comprise a combination of two possible gas components, in which case an additional m(mxe2x88x921)/2 possibilities arise. Thus, m+m(mxe2x88x921)/2=m(m+1)/2 different calibrating gases are obtained, whose number is now large enough to guarantee a reliable calibration.
The corresponding n measured values of the n sensors are passed on to the evaluating unit for each calibrating gas.
The measured values of a calibrating gas form an n-dimensional measurement point, which can be considered to be the vector in the n-dimensional vector space of all possible measured value configurations from n measured values. The number of measurement points thus obtained equals the number of calibrating gases selected. An m-dimensional subvector space is sought for these measurement points in the n-dimensional vector space of all possible measurement points, i.e., of all possible measured value configurations, which comprises in the best approximation all measurement points of the calibrating gases. This is to be understood such that a subvector space is constructed, e.g., such that the sum of the distances between the measurement points belonging to the calibrating gases and the subvector space is as small as possible. A tolerance range, which surrounds the subvector space, is determined for the subvector space. No restricting conditions are attached, in principle, to the tolerance range. However, the tolerance range is preferably selected to be such that it comprises the measurement points whose distance from the subvector space can still be explained with the usual measurement inaccuracies, i.e., noises. In light of the evaluation by calculation, it is advantageous to set the tolerance range as the range of the measurement points in the vector space of all possible measurement points whose distance from the subvector space determined is below a preset fixed distance value.
After the conclusion of the calibration operation, the gas sample is introduced into the gas-measuring device, and n values, which will yield an n-dimensional measurement point, are measured by means of the n sensors.
If this measurement point is located within the tolerance range, it is classified by the evaluating unit as plausible; if the measurement point is located outside the tolerance range, it is classified by the evaluating unit as implausible.
An essential feature of the process is based on the fact that the measured value configurations are considered to be measurement points in a vector space, because simple criteria can thus be formulated by calculation, by means of which criteria it can be decided whether the measurement point belonging to a gas sample is to be classified as plausible, i.e., as a measurement point that has a sufficiently short distance from the set of the measurement points in the vector space, which are obtained from the calibration data, or whether the measurement point belonging to the gas sample is to be classified as implausible, i.e., as a measurement point that is located at an excessively great distance from the set of measurement points obtained from the calibration data. How a xe2x80x9cset of measurement pointsxe2x80x9d and how a xe2x80x9cdistancexe2x80x9d are to be selected is obvious from the vector space structure. The advantage of the process is that immediately after a measurement and with comparatively simple methods, a measuring result can be discarded if it is to be considered to be implausible based on the measured values. This would be the case, e.g., when one of the sensors measures a greatly increased value that is unrealistic in connection with the other values due to its cross sensitivity to a gas component not taken into account during the calibration. Such an example is a sensor that is defective and hardly responds, so that only extremely low values are measured. The evaluating unit classifies the corresponding measurement point as implausible in both cases, and the user can track down the cause of the disturbing effect.
Electrochemical gas sensors are used as the sensors in a preferred embodiment.
A design of the process with n=3 sensors and m=2 possible gas components is of particular significance for practice. These are, e.g., chlorine (Cl2) and sulfur dioxide (SO2). The concentrations of the possible gas components chlorine and sulfur dioxide can thus be determined in one gas sample after the conclusion of the calibration operation, and implausible measuring results are immediately recognized and their cause can be explored.
If a measurement point has been classified by the evaluating unit as plausible or implausible, this result is communicated in a preferred embodiment to the user via a display unit. In case of implausible measurement points, it would be possible, e.g., to send a warning to the user.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.